The invention relates generally to fluid-cooled rotating members and particularly to dynamoelectric machines having a rotor with superconducting windings.
General concepts for superconducting dynamoelectric machines are known and are of growing interest because of the need to maximize machine efficiency and reduce the size of machines of given ratings. One area of interest is the application of superconducting technology to alternating current generators. In one form, such a generator consists of a stator with a conventionally conductive armature winding and a rotor with superconducting field coils or windings. To attain superconductivity in available conductors, such as those consisting of niobium and titanium alloy, requires that the maximum winding temperature be about 9.degree. to 11.degree. K. To achieve such temperatures, a cryogenic coolant, such as liquid helium having a temperature of 4.2.degree. K, is circulated in intimate thermal relation with the conductors of the windings. Such a coolant is expensive and the thermal margin between the coolant temperature and the critical temperature of the superconductive winding is relatively small so that important design criteria are to minimize the use of coolant and to minimize heat conduction into the winding from external structural elements that may be at or near ambient temperature.
Previously proposed arrangements for a rotor with superconducting windings contemplate placing the rotor windings inside a torque tube within a dewar type vessel to minimize heat transfer. The structural elements at the periphery of the tube support the field poles and windings and transmit machine torque to a shaft and rotor drive coupling. Such structural elements can cause a large heat conduction into the cold region. One such machine of the prior art is that disclosed in Analysis of the Application of Superconductivity to Commercial Electric Power Generation by P. Thullen, a doctoral thesis at the Massachusetts Institute of Technology, dated June, 1969. In accordance with this publication, the conduction heat losses from the periphery of the torque tube are intended to be reduced by passing the coolant exhausted from the field winding over, through or in contact with the support structure. Because the support structure is rotating, and the coolant is therefore rotating, body forces exist in the coolant that cause convection currents to flow in the structural cooling passages. These convection currents tend to equalize the coolant temperature in the structure thus increasing heat flow to the low temperature region of the machine. The convection currents are proportional to the rotational speed, the distance from the axis of rotation and the differences that occur in coolant density as a result of rotation.
The above referred to thesis recognizes the occurrence of convection currents and intends to avoid them by placing baffles in the exhaust coolant passages. This expedient, and others for avoiding convection currents in the rotating coolant, is only partially successful and it is desirable to further improve the means for cooling the support structure.